tests/qtest/meson.build: Bump the boot-serial-test timeout to 4 minutes

When running with TCI, the boot-serial-test can take longer than 3 minutes:

 https://gitlab.com/qemu-project/qemu/-/jobs/5890481086#L4774

Bump the timeout to 4 minutes to avoid CI failures here.

Message-ID: <20240115071146.31213-1-thuth@redhat.com>
Reviewed-by: "Daniel P. Berrangé" <berrange@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>

tests/qtest/meson.build

@@ -8,7 +8,7 @@ slow_qtests = {
   'test-hmp' : 240,
   'pxe-test': 600,
   'prom-env-test': 360,
-  'boot-serial-test': 180,
+  'boot-serial-test': 240,
   'qos-test': 120,
 }